Due to the economics of the ink business today, it is highly desirable to be able to process pigments into high-solids concentrates. Typically, such concentrates would contain from 30 to 60% pigment. Ideally, they could be let down in a wide variety of vehicles in order to produce a wide range of finished inks.
The economic advantages offered by such concentrates are twofold. First, they would greatly reduce the inventory of intermediates which must be carried by an ink plant. Only one intermediate dispersion would be needed for each pigment versus the several which are now typically used. Second, energy consumption would be considerably reduced. The higher the pigment concentration at which dispersion can be effected, the less the total volume of material that must be processed through a mill and therefore the less total energy consumed in processing the pigment.
Traditionally, vehicles for lithographic inks are composed of alkyd resins and/or rosin derivatives and/or hydrocarbon resins, together with high-boiling hydrocarbon solvent. It is known in the art that satisfactory dispersions containing 30 to 60% pigment cannot be prepared using such traditional vehicle components alone. Usually mixtures at such solids levels cannot be processed in standard milling equipment. In the few cases where they can be processed, they yield dispersions with unacceptable rheological properties, i.e. dispersions which cannot be pumped or otherwise conveniently transferred from one vessel to another. Such dispersions usually also show inferior color development and poor aging stability.
Recently attempts have been made to overcome the above-described problems by using compounds which are better dispersants for pigments than are the traditional alkyd resins and rosin derivatives. For example, U.S. Pat. No. 4,224,212 describes the use of dispersing agents attained by reacting a poly (lower alkylene) imine with a polyester having free carboxylic acid groups to form reaction products containing at least two polyester chains attached to each poly (lower alkylene) imine chain. Preferred polyesters are the polyesters of an hydroxy carboxylic acid of the formula HO--R--COOH where R is a divalent aliphatic radical containing at least 8 carbon atoms in which there are at least 4 carbon atoms between the hydroxy and carboxylic acid groups. Preferred polyesters are also the polyesters formed from a mixture of aforementioned hydroxy carboxylic acids with a carboxylic acid which is free from hydroxy groups.
U.S. Pat. No. 4,415,705 describes the use of rosin derivatives as dispersants. These dispersants are attained by reacting a poly (lower alkylene) imine having a molecular weight of 1,000 to 15,000 with a polyester obtained by esterifying hydroxy stearic acid or its oligomer, with tall oil rosin. This patent states that such dispersants have been found to be superior to products made from wood rosin or gum rosin.